Bridging and Fixing Metal-Organic Cages

Despite their well-defined and intrinsic porous structures, metal-organic cages (MOCs) readily lose their crystalline arrangement upon solvent exchange and desolvation, which significantly reduces their porosity. Herein, we report a novel bridging and fixing strategy for retaining the ordered arrangement of MOCs. In the bridging process, electrophilic aromatic substitution reactions involving the 4,6-dihydroxy-1,3-benzenedicarboxylate (m-DOBDC2-) ligands of cuboctahedral MOCs and formaldehyde connect the cages by methylene bridges, which are among the shortest linkers. In the subsequent fixing process, pore-filling and coordinating solvents are exchanged with or removed by low surface tension, noncoordinating solvents (such as mesitylene) in which MOCs are poorly soluble. This mild activation process avoids altering the cage arrangement, thereby further increasing the porosity of the bridged MOCs. The general applicability of this fixing process is demonstrated using cuboctahedral MOCs with different functional groups. As a result, after the fixing process, MOP-1-H displays the highest surface area of 1090.7 m2/g among Cu-paddlewheel-based MOCs. This study extends the potential applications of MOCs as functional porous materials by providing a simple and pragmatic method for preserving their porosities.